Statement A: According to the (n+l) rule, if two orbitals have the same (n+l) value, the orbital with the lower value of n has lower energy. This statement is true.
Statement B: As the atomic number increases, the effective nuclear charge (Zeff) increases. The energy of an orbital is proportional to −n2Zeff2. Thus, the energy becomes more negative, meaning the energy of the orbitals in the same subshell decreases with an increase in atomic number. This statement is false.
Statement C: The size of an orbital increases with an increase in the principal quantum number n. Therefore, the size of the 2px orbital is less than the size of the 3px orbital. This statement is true.
Statement D: The number of radial nodes is given by the formula n−l−1.
For 5f: 5−3−1=1
For 6s: 6−0−1=5
For 4d: 4−2−1=1
For 5p: 5−1−1=3
For 5d: 5−2−1=2
Since the 5d orbital has exactly 2 radial nodes, the statement that none of the orbitals have 2 radial nodes is false.
Therefore, only statements A and C are true.
Answer: A and C only